Coherent Diffractive Imaging Technique Research Articles

Perspective on the Use of Coherent Diffraction Imaging as a Tool for High Resolution Materials Characterization

Coherent diffraction imaging (CDI) is an emerging technique that is currently used mainly in the field of physics. An opportunity is present for materials scientists to make use of this technique in order to solve current and future materials-centric problems. In addition to introducing the scope of the section in this article, the readers are also given backgrounds for the authors and how each paper fits together. Finally, a brief review of the inaugural symposium held at the TMS 2013 Annual Meeting is provided along with an invitation for the next symposium scheduled for 2015. Although the concept of CDI has been around since the 1950s, experimental realization of this technique occurred only about 15 years ago. Through a combination of the oversampling method and iterative phase retrieval algorithms, high-resolution images can be obtained through the inversion of diffraction patterns from both crystalline and noncrystalline materials. This allows imaging and characterization through the use of x-rays to occur for a broad range of materials including nanoparticles, strained crystals, biomaterials, and cells. Unique characterization abilities such as threedimensional strain mapping and nondestructive three-dimensional tomographic imaging highlight the emerging importance of CDI in the field of materials science. In this series of papers, current and potential applications of CDI techniques to solve problems in materials science are explored as a way to foster increased interaction between materials scientists and physicists, the predominant developers of CDI. The first paper in this series is titled ‘‘From Grain Boundaries to Single Defects: A Review of Coherent Methods for Materials Imaging in the X-ray Sciences.’’ In this paper, Brian Abbey (La Trobe University, Melbourne, Australia, and Melbourne Centre for Nanofabrication, Melbourne, Australia) provides a broad overview of the various techniques within the field of CDI. Brief descriptions of the governing physics as well as limits of the techniques such as phase contrast tomography, plane wave CDI, ptychographic CDI, diverging beam CDI, reduced coherence CDI, coherent x-ray microbeam diffraction, and Bragg CDI are reported. The author also discusses current and potential uses for each technique within the materials science field. Dr. Abbey, in addition to the previously listed appointments, is also a member of the Micro Materials Characterization Beamline development team for the Australian Synchrotron. The second paper in this series is titled ‘‘X-Ray Coherent Diffraction Imaging of Nanomaterials.’’ Ross Harder (Advanced Photon Source, Argonne, IL, USA) and Ian K. Robinson (University College London, London, England) explore the use of Bragg CDI to image structure in nanosized barium titanate crystals in conjunction with local strain measurements with high sensitivity. Harder manages the 34-ID-C beamline at the Advanced Photon Source and welcomes user proposals that will continue to push the envelope in CDI’s abilities and applications, especially in the field of strain mapping. The final paper in this series is titled ‘‘Studies of Materials at the Nanometer Scale using Coherent X-Ray Diffraction Imaging.’’ Richard Sandberg (Los Alamos National Laboratory, Los Alamos, NM, USA), Zhifeng Huang, Rui Xu, Jose Rodriguez, and John Miao (all from the University of California Los Angeles, Los Angeles, CA, USA) explore the use of CDI in imaging materials as varied as biological cells and GaN quantum dot nanoparticles. The use of CDI is discussed in the context of both beamline-based CDI and table-topsource-based CDI. Sandberg’s primary research focus involves the development of table-top sourJohn Carpenter, chair of and JOM advisor for the Materials Characterization Committee of the TMS Extraction & Processing Division, and Richard Sandberg coordinated the topic Coherent X-ray Diffraction Imaging as a Characterization Tool in this issue. JOM, Vol. 65, No. 9, 2013

Bragg x-ray ptychography of a silicon crystal: Visualization of the dislocation strain field and the production of a vortex beam

We experimentally demonstrate the visualization of nanoscale dislocation strain fields in a thick silicon single crystal by a coherent diffraction imaging technique called Bragg x-ray ptychography. We also propose that the x-ray microbeam carrying orbital angular momentum is selectively produced by coherent Bragg diffraction from dislocation singularities in crystals. This work not only provides us with a tool for characterizing dislocation strain fields buried within extended crystals but also opens up new scientific opportunities in femtosecond spectroscopy using x-ray free-electron lasers.

Ptychographic Imaging at the Swiss Light Source

One of the high-resolution X-ray microscopy techniques currently being developed at the Swiss Light Source (SLS), as in other places, is based on ptychographic imaging. Ptychography is a coherent diffractive imaging (CDI) technique that bypasses the need for high-numerical-aperture X-ray optics by “phasing” diffraction patterns [1]. Measured in the Fraunhofer far field, the intensity distribution of coherent light scattered by a specimen is easily related to the wave field directly past the specimen by little more than a Fourier transform. However, only intensities are measurable, and the all-important phase information needs to be “retrieved” through analytical means. CDI is insensitive to aberrations and manufacturing limitations of optics and can yield highly resolved images with quantitatively interpretable contrast.

High-contrast imaging for weakly diffracting specimens in coherent diffraction imaging

Coherent diffraction imaging (CDI) and ptychography techniques bypass the difficulty of having highquality optics in X-ray microscopy by using a numerical reconstruction of the image that is obtained by inverting the diffracted intensity recorded by a charge-coupled device array. However, the reconstruction of the image from the intensity data obtained from a weakly diffracting specimen is known to be difficult because of the obvious reduction in signal-to-noise ratio (SNR). In this case, the specimen only slightly modifies the probe diffraction pattern, resulting in difficulty in the identification of the detailed structure of the specimen from the reconstructed image because of the poor contrast and sharpness of the image. To address this situation, a modification in the image retrieval algorithms used in the iterative reconstruction of the image is suggested. This modification should double the presence of high spatial frequencies in the diffraction pattern to enhance the contrast and edge detection in existing imaging techniques.

高分解能・高感度X線タイコグラフィー

X-ray ptychography, which is one of the coherent X-ray diffractive imaging techniques, allows us to observe extended objects with a high-spatial resolution and a wide field of view. We have developed high-resolution and high-sensitivity X-ray ptychography using the high-intensity X-ray beam focused by total reflection mirrors at SPring-8. In addition, we have applied it to the elemental mapping of metallic nanoparticles and the imaging of the dislocation strain fields of a silicon film. This method will be available to observe various materials and biological specimen.

Sparsity-based single-shot subwavelength coherent diffractive imaging

Coherent Diffractive Imaging (CDI) is an algorithmic imaging technique where intricate features are reconstructed from measurements of the freely diffracting intensity pattern. An important goal of such lensless imaging methods is to study the structure of molecules that cannot be crystallized. Ideally, one would want to perform CDI at the highest achievable spatial resolution and in a single-shot measurement such that it could be applied to imaging of ultrafast events. However, the resolution of current CDI techniques is limited by the diffraction limit, hence they cannot resolve features smaller than one half the wavelength of the illuminating light. Here, we present sparsity-based single-shot subwavelength resolution CDI: algorithmic reconstruction of subwavelength features from far-field intensity patterns, at a resolution several times better than the diffraction limit. This work paves the way for subwavelength CDI at ultrafast rates, and it can considerably improve the CDI resolution with X-ray free-electron lasers and high harmonics.

Chemical Contrast in Soft X-Ray Ptychography

The unique strengths of x-ray microscopy are high penetration depth and near-edge resonances that provide chemical information. We use ptychography, a coherent diffractive imaging technique that disposes of the requirement for isolated specimens, and demonstrate resonant imaging by exploiting resonances near the oxygen K edge to differentiate between two oxygen-containing materials. To highlight a biological system where resonant ptychography might be used for chemical mapping of unsliced cells, reconstructions of freeze-dried Deinococcus radiodurans cells at an energy of 517 eV are shown.

Coherent imaging at the Diamond beamline I13

AbstractThe Diamond beamline I13 for imaging and coherence applications is dedicated to hard X‐ray imaging on the micro and nano‐lengthscale. Two independent stations will be operated in a separate building at a distance of 250 m from the source. The imaging branch will perform in‐line phase contrast imaging and tomography over a large field of view in the 6–35 keV energy range. In addition it will be possible to switch to full‐field microscopy with 50 nm spatial resolution. Other microscopies will be developed according to the scientific needs. Resolution beyond the limitations given by the detector and X‐ray optics will be achieved on the ‘coherence’ branch. Techniques working in the far field such as Coherent X‐Ray Diffraction and other Coherent Diffraction Imaging techniques such as ptychography will be implemented. Typical applications cover different fields such as bio‐medicine, materials science, geo‐ and astrophysics or even cultural heritage. The beamline hosts a number of innovative features such as the so‐called ‘mini‐beta’ layout for electron optics in the storage ring or new concepts for beamline instrumentation. The stations will be operational in 2011.

Coherent x-ray diffraction imaging of paint pigment particles by scanning a phase plate modulator

We have implemented a coherent x-ray diffraction imaging technique that scans a phase plate to modulate wave-fronts of the x-ray beam transmitted by samples. The method was applied to measure a decorative alkyd paint containing iron oxide red pigment particles. By employing an iterative algorithm for wave-front modulation phase retrieval, we obtained an image of the paint sample that shows the distribution of the pigment particles and is consistent with the result obtained from a transmission x-ray microscope. The technique has been experimentally proven to be a feasible coherent x-ray imaging method with about 120 nm spatial resolution and was shown to work well with industrially relevant specimens.

Phase-Diverse Coherent Diffractive Imaging: High Sensitivity with Low Dose

This Letter demonstrates that coherent diffractive imaging (CDI), in combination with phase-diversity methods, provides reliable and artefact free high-resolution images. Here, using x rays, experimental results show a threefold improvement in the available image contrast. Furthermore, in conditions requiring low imaging dose, it is demonstrated that phase-diverse CDI provides a factor of 2 improvement in comparison to previous CDI techniques.

Optical ptychography: a practical implementation with useful resolution

Quantitative phase microscopy offers a range of benefits over conventional phase-contrast techniques. For example, changes in refractive index and specimen thickness can be extrapolated and images can be refocused subsequent to their recording. In this Letter, we detail a lensless, quantitative phase microscope with a wide field of view and a useful resolution. The microscope uses the recently reported coherent diffractive imaging technique of ptychography to generate images from recorded diffraction patterns.

Coherent diffraction surface imaging in reflection geometry

We present a reflection based coherent diffraction imaging method which can be used to reconstruct a non periodic surface image from a diffraction amplitude measured in reflection geometry. Using a He-Ne laser, we demonstrated that a surface image can be reconstructed solely from the reflected intensity from a surface without relying on any prior knowledge of the sample object or the object support. The reconstructed phase image of the exit wave is particularly interesting since it can be used to obtain quantitative information of the surface depth profile or the phase change during the reflection process. We believe that this work will broaden the application areas of coherent diffraction imaging techniques using light sources with limited penetration depth.

Structural inhomogeneity in silicon-on-insulator probed with coherent X-ray diffraction

We report our research on X-ray micro-beam diffraction and coherent X-ray diffractive imaging techniques to study structural inhomogeneities in Silicon-On-Insulator continuous plain wafers. Inhomogeneities were measured quantitatively and attributed to limitations of the manufacturing process. 3-dimensional image reconstructions were performed by using our Error-Reduction and Hybrid-Input-Output iterative algorithms. These revealed images of the focussed X-ray beam passing through the active layer of the wafer.

Hard and soft X-ray microscopy and tomography in catalysis: bridging the different time and length scales

X-ray microscopic techniques are excellent and presently emerging techniques for chemical imaging of heterogeneous catalysts. Spatially resolved studies in heterogeneous catalysis require the understanding of both the macro and the microstructure, since both have decisive influence on the final performance of the industrially applied catalysts. A particularly important aspect is the study of the catalysts during their preparation, activation and under operating conditions, where X-rays have an inherent advantage due to their good penetration length especially in the hard X-ray regime. Whereas reaction cell design for hard X-rays is straightforward, recently smart in situ cells have also been reported for the soft X-ray regime. In the first part of the tutorial review, the constraints from a catalysis view are outlined, then the scanning and full-field X-ray microscopy as well as coherent X-ray diffraction imaging techniques are described together with the challenging design of suitable environmental cells. Selected examples demonstrate the application of X-ray microscopy and tomography to monitor structural gradients in catalytic reactors and catalyst preparation with micrometre resolution but also the possibility to follow structural changes in the sub-100 nm regime. Moreover, the potential of the new synchrotron radiation sources with higher brilliance, recent milestones in focusing of hard X-rays as well as spatiotemporal studies are highlighted. The tutorial review concludes with a view on future developments in the field of X-ray microscopy that will have strong impact on the understanding of catalysts in the future and should be combined with in situ electron microscopic studies on the nanoscale and other spectroscopic studies like microRaman, microIR and microUV-vis on the macroscale.

Coherent Diffractive Imaging Using Phase Front Modifications

We introduce a coherent diffractive imaging technique that utilizes multiple exposures with modifications to the phase profile of the transmitted wave front to compensate for the missing phase information. This is a single spot technique sensitive to both the transmission and phase shift through the sample. Along with the details of the method, we present results from the first proof of principle experiment. The experiment was performed with 6.0 keV x rays, in which an estimated spatial resolution of 200 nm was achieved.

Keyhole coherent diffractive imaging

A powerful coherent diffractive X-ray imaging technique could enable next generation synchrotrons and free-electron lasers to study much larger samples than previously thought possible.